Acid bonding nonwoven fabrics

ABSTRACT

A PROCESS FOR MAKING A BONDED NONWOVEN FABRIC FROM A BLEND OF NYLON FIBERS AN FIBERS OF ANOTHER KIND OF MATERIAL. THE FIBERS ARE FORMED INTO A WEB WHICH IS TREATED WITH A CONCENTRATED SOLUTION OF A STRONG ACID WHICH AFFECTS THE NYLON BUT DOES NOT SUBSTANTIALLY AFFECT THE OTHER MATERIAL. THE NYLON IS SOFTENED AND SWOLLEN BY THE CONCENTRATED ACID. THEN, THE WEB IS CONTACTEDWITH WATER WHICH DILUTES THE CONCENTRATED ACID AND COAGULATES THE NYLON. SUBSEQUENTLY, THE WEB IS SUBJECTED TO PRESSURE. THE ACID TREATMENT MAKES THE NYLON MORE SUSCEPTIBLE TO PRESSURE BUT DOES NOT HAVE THIS EFFECT ON THE FIBERS OF THE OTHER MATERIAL. WHEN PRESSURE IS APPLIED, THE NYLON FLOWS AND CAUSES THE OTHER FIBERS TO BE BONDED TO EACH OTHER.

United States Patent Office 3,647,591 Patented Mar. 7, 1972 ABSTRACT OFTHE DISCLOSURE A process for making a bonded nonwoven fabric from ablend of nylon fibers and fibers of another kind of material. The fibersare formed into a web which is treated with a concentrated solution of astrong acid which affects the nylon but does not substantially affectthe other material. The nylon is softened and swollen by theconcentrated acid. Then, the web is contacted with water which dilutesthe concentrated acid and coagulates the nylon. Subsequently, the web issubjected to pressure. The acid treatment makes the nylon moresusceptible to pressure but does not have this effect on the fibers ofthe other material. When pressure is applied, the nylon flows and causesthe other fibers to be bonded to each other.

The present invention relates to the manufacture of a bonded nonwovenfabric and more specifically to fabrics useful as a substrate forartificial leather. In accordance with the present invention, a nonwovenfiber web composed of unbonded fibers including nylon fibers and othersis treated with strong acid to selectively affect the nylon fibers, andthe web can then be calendered to cause the nylon fibers to further bondthe web into a more cohesive fabric.

Leather is composed of a plurality of collagen fibers bound together byreticular tissue which forms a network between the collagen fibers.Therefore, in the manufacture of artificial leather, an effort has beenmade to substitute textile fibers for the collagen fibers and asynthetic polymer for the reticular tissues.

There are certain characteristics of leather which must be reproduced ina leather replacement material. For shoe uppers, which consume more thanhalf of the hides used in the U.S., these characteristics includeoptimum values of density, relative density, uniformity of density,permeability, roll, break, piping, crease resistance, suppleness,forrnability (lastability) and dimensional stability to low order stressand strain.

Density is the weight of a given volume of a material and is expressedin grams per cubic centimeter according to the equation.

Density (gm/cc.)

Weight (ounces per square yard) X 1.33 Thickness (mils) (RelativeDensity) Weight (ounces per square yard) X 1.33 Average specific gravityof components X thickness (mils) A relative density, for example, of .51indicates a pore space or porosity of .49 (49%). Porosity is theproportion of the volume of a material occupied by interstices.

Relative density, together with density, characterizes the degree ofcompactness of a fibrous sheet structure.

Density uniformity or uniform density.This property relates to theevenness of interstice and fiber distribution within the structure. Amethod of measuring this property is described in US. Pat. 2,958,113. Inthat method a fiber web is placed between a light source moving in afour-inch line and pulsed at 60 cycles per second and a photocell whichfeeds its pulsed input into a display cathode ray oscilloscope. Theprojection of the oscilloscope represents the varying intensity of lightas it passes through the batt along its path. The top peaks representsareas of no light transmission through the batt. The bottom peaks denotelight transmission through the batt. Non-uniform nature of the batt willbe represented by wide fluctuations, particularly in the bottom peaksand widespread between top and bottom peaks. Relatively uniformfluctuations between top and bottom peaks demonstrate uniformity of theproduct. Relatively narrow spread between top and bottom peaks indicateshigh covering power.

Another means to determine density uniformity is examination of crosssections of the fibrous sheet under a microscope at about 30magnifications.

Preferably a flexible film is coated or laminated to the surface of afibrous sheet material and the texture and appearance of the coatingupon stretching is examined as to the development of irregularitiesproduced by nonuniform density of the fibrous sheet substrate.

Permeability is the ability to allow vapors and liquids to pass throughthe interstices or pores. It denotes that the interstices of a materialare interconnected and are not iso lated and sealed off one from theother as in buoyant foamed polystyrene. Fibrous sheet materials of up to.75 relative density, i.e. 25% pore space, are extremely permeable tovapors and liquids, provided they are unsaturated or uncoated andpossess uniform density. A preferred test method for permeability isdescribed in US. Pat. 2,723,935.

Roll as related to a flexible sheet material, is the character of itsresistance to the rolling or flexure of a rear flat bend or crease. Itis tested by producing a sharp (small radii) bend of and subsequentfiexure of the near flat bend or crease by moving the faces of thefolded sheet across one another back and forth in a direction normal tothe bend or crease line. Sheet materials with good roll offer even anduniform resistance to the rolling or flexure of a sharp bend or nearfiat crease. This is most practically evaluated by folding a small sheetof the material between the extended fingers of both hands and thenmoving the hands back and forth as when rubbing the palms of the handstogether.

Break, as related to a flexible sheet material, is the continuity ofstructure and appearance of the material on the concave and convex sideof a sharp (small radii) bend as it is subjected to flexure as describedabove when testing or examining for roll characteristics. A markedchange in continuity of structure and appearance upon roll testing isconsidered poor break. When one breaks in a pair of shoes, theappearance of the creases and folds at the instep allows one to judgethe break characteristics of the leather or flexible sheet material ofthe shoe upper.

Piping is a descriptive term also used to denote the degree of structureand appearance of change along the concave line of a sharp (small radii)bend or fold of leather or a flexible sheet material. This is observedwhen testing for roll and break. The structure of a flexible sheetmaterial is subjected to considerable compressive forces on the concaveside of a shape (small radii) bend or fold. If the structure collapses,deep creases develop in conjunction with pipelike protrusions of thestructure 3 between creases. This is excessive piping and indicates poorroll and break.

Crease resistance is the ability of a flexible sheet material or leathernot to collapse or develop deep creases when subjected to theconsiderable compressive forces of a sharp (small radii) bend or fold.

Suppleness is principally considered to be softness or lack ofstiffness. Stiffness (or softness) may be tested on a Tinius-Olesenmachine in accordance with ASTM 1388-55T.

Formability or lastability is the ability of a leather or sheet materialto be forcibly drawn or stretched into a new configuration withoutexhibiting the memory or desire to return to its original configurationas is characteristic of elastic sheet material. An example is theforming of a shoe upper toe from a fiat sheet. In order for a materialto be formable it must possess a very uniform structure and itscomponents must be able to slip upon each other and assume a stable newconfiguration.

Dimensional stability to low order stress and strain is the ability ofthe fibrous sheet material to be rolled and unrolled, saturated withaqueous or solvent saturant systems, heated, coated etc. withoutexcessive elongation, stretching or necking down upon being subjected tothe normal tensions of such handling.

Sheet materials ideally suited for shoe upper leather, suede leather,wearing apparel and the like, must possess properties of suppleness, andgood roll, and break. Therefore, many solid compositions of rubber andplastic sheets have been considered as artificial leather because theypossess the correct properties of suppleness and drape, roll and break.However, they are not well suited for use as shoe uppers, wearingapparel and the like because they do not normally possess the propertiesof leatherlike feel, permeability, porosity and the ability to draw,last, or be formed without memory or desire to return to their originaldimension. Non-solid all fibrous sheet materials like felts or otherneedle-punched and shrunk materials also have been considered becausethey possess many of the desirable properties referred to above. Theyare not suitable themselves, however. Consequently, it has beensuggested that they be subjected to saturation or impregnation by arubber or plastic material, with or Without adhesion of the impregnantto the fibrous sheet, in order to obtain all of the properties desiredin a shoe upper, suede leather or wearing apparel material.

For some types of artificial leather, e.g. intended to be used as shoeuppers, it has been found that porosity and gas permeability are notessential characteristics. In such cases, it has become common toutilize the artificial leather material made by coating a plastic, suchas polyvinyl chloride, onto a woven fabric. The present invention isconcerned with the manufacture of a nonwoven fabric which is useful as asubstrate for making artificial leather of this type.

In accordance with the present invention, a nonwoven fabric is made froma fiber blend of which a portion of the fibers comprise nylon. Theblended fibers are formed into a web which preferably is needle punchedand the web is then immersed in strong, concentrated acid. This tends tosoften and flow the nylon fibers, and the web is then immersed in Waterwhich coagulates the nylon. The web is bonded at this point but not aswell or tightly as it is after also calendering. Next the web isneutralized with an alkali, or the nylon can be coagulated andneutralized simultaneously by passing the web from the acid bathdirectly into the alkali bath, followed by washing and, if necessary,drying. The Web can then be calendered or compressed while hot enough tocause the treated nylon to flow. The acid treatment reduces thetemperature at which the nylon flows under pressure whereas the otherfibers in the blend are not affected by the acid. Therefore, at thetemperature applied when the fibers are pressed, the nylon isselectively flowable while the other fibers remain essentiallyunaffected .so that the nylon becomes a more efiicient bonding agentwhich holds the other fibers together.

The nylon fibers which are used in the present invention are fibers ofpolyamides which are condensation products containing recurring amidegroups as integral parts of the main polymer chains. They are made bycondensation of amino acids or lactams derived from them, or fromdiamines and dibasic acids, or salts or the like made from them. Thepolyamides are fiber-forming which means that they are of relativelyhigh molecular weight. The invention is particularly concerned with theuse of nylon 6 and/or nylon 6,6. It also may be used with nylon 6,10,although this polymer requires stronger treatment. Nylon 6 is apolyamide derived by polycondensation of 6-amino-caproic acid or apolyamide forming derivative thereof such as caprolactam. Nylon 6,6 isthe polycondensate of adipic acid and hexamethylene diamine, orpolyamide derivatives thereof, particularly the salt formed by reactionof hexamethylene diamine and adipic acid. Nylon 6,10 is a polyamidederived by polycondensation of hexamethylene diamine and sebacic acid orpolyamide forming derivatives thereof. Copolyamides especially of theaforesaid amide forming materials also may be used.

As fibers useful for blending with the nylon fibers, there may bementioned polyester, polyolefin, e.g. polypropylene, glass, cellulosefibers such as cotton and rayon, polyvinyl chloride, Saran and the like.Wool might be used if the acid is, say, sulfuric acid which does notaffect it, even though it is a kind of polyamide. The criteria forselection of other fibers is that they be relatively unaffected by theacid treatment '(and that they preferably have a softening point higherthan the nylon after the nylon has been treated with acid).

The fibers utilized in accordance with the invention may be oriented orunoriented, although it is normally preferred that at least thenon-nylon fibers be oriented to increase their tensile strength. Thefibers ordinarily will be 1.0 to 15 denier and about 1 /2 inches long.In the blend, the nylon may constitute about 20-80 percent by weight.The fibers may be blended together using any conventional fiber blendingtechnique.

The fibers in the blend are formed into a nonwoven fiber web. This termis used to describe a web such as a batting or similar materialcomprising fibers arranged at random 'but not bonded to each other. Aweb may be formed, for example, by driving the fibers, preferably whiledry, onto a moving screen with air blowing the fibers along and/or withsuction applied through the screen. A Web also may be formed using acard which tends to produce a higher degree of alignment of the fibersin the machine direction.

The web may be built up to greater thickness than originally formed asdescribed above, by placing successive layers of theweb on top of eachother, for instance in a cross laying machine; the cross laying ofseveral layers tends to cancel out some of the nonuniformities inindividual layers. It also is preferred to combine the nonwoven fiberweb by needle punching with a carrier layer which is a sheet materialhaving greater dimensional stability than the nonwoven fiber web itself.This holds the web together during subsequent treatment. Almost any kindof carrier material may be used in this embodiment. For instance, it maybe a woven scrim, i.e. an open mesh plain woven fabric of cotton invarious weights and constructions. The carrier layer also may be a sheetmaterial of any type, for example, a plastic film, a plastic foam, suchas polyurethane, or a ligated or previously bonded nonwoven Web.Typically, the carrier will have a weight of 0.5-3.0 ounces per squareyard. It is possible that the carrier layer may be composed of nylon,for instance a nylon scrim of film, which is affected by the acidbonding treatment in accordance with the present invention, increasingthe bond strength of the fabric and permitting the acid treatment toeliminate orange peel which otherwise might be caused by the carrierlayer.

In lieu of a reinforcing carrier, the web may be bonded lightly byconventional techniques, For example, the web can be impregnated with arelatively dilute latex of binder, such as cross-linkable thermoplasticacrylic polymer or rubber, and the binder may be cross-linked byheating. Preferably the amount of binder is relatively small, up toabout by weight of the fibers, and insufficient to coat the nylon fiberssubstantially.

The nonwoven fiber web. preferably laminated to the carrier layer, isfirst subjected to needle punching. This is a process whereby aplurality of needles, ordinarily having barbs projecting from them, arepushed into the fabric and withdrawn repeatedly. Needle punching tendsto densify the web, and also may cause some shortening of the fibers andredistribution of the fibers among themselves, increasing the internalstrength of the web. The needle punch density used in accordance withthe present invention will range from 1000-20,000 punches per squareinch, preferably about 3000 punches per square inch. In terms ofleather-like properties, needle punching increases density, improvesroll and break, and possibly other properties as well. However, needlepunching tends to form pock-marks on the surface of the web, which showthrough a superimposed polymer coating when the product is used inartificial leather. One of the advantages of the subsequent treatmentsin accordance with the present invention is that they remove thesemarks.

The needle punched nonwoven fiber web then is contacted with strongacid. Contact may be accomplished in many ways such as spraying the acidonto the web, padding or simply immersing the web into the acid.

A variety of acids may be used for the treatment. The acid should be astrong one, that is having a dissociation constant in water at 20 C. ofor greater. The acid should be water soluble, and preferably has adissociation constant of 10- or greater. Formic acid has been used, butstill stronger acids are preferred, notably sulfuric, nitric,phosphoric, hydrochloric and fluoboric acids. These acids are applied tothe fabric from an aqueous solution, usually containing about 25-75 byweight of acid, the balance being water. The exact concentration'varies, depending upon the strength of the acid. In general, the Weakeracids are used in higher concentration and the stronger acids in lowerconcentration. For instance, sulfuric acid may be used at as low aconcentration as 30%, whereas formic acid requires a concentration ofthe order of 75% or more. The exact concentration to be used can bedetermined by a simple test. Samples of a particular acid in water atdifferent concentrations are made up and samples of nonwoven fiber webcontaining nylon fibers are immersed in the solutions. The samples,after immersion for 10*20 seconds, are withdrawn, immersed in water atroom temperature, dried and tested for stiffness. The concentration ofacid which gives the desired degree of stiffness in the fabric is theconcentration to be used. If the stiffness is not increased, theconcentration is too low. On the other hand, if the stiffness isincreased excessively, the concentration is too high. The acid treatmentalso produces a change in the fabric which can be seen by holding thefabric up to light, i.e. a change in its optical density. Thus, theminimum concentration which produces the visible change can be taken asthe minimum concentration to be used.

The acid treatment need be continued only for a few seconds. In general,a contact time sufficient for the acid to fully penetrate the Web isdesired, but long contact time is not necessary. The effect of the acidtreatment depends primarily on the strength and concentration of acid,rather than the duration of the treatment. 3-5 seconds contact timeshave been sufiicient, and little is gained by continuing the treatmentfor more than about 10 seconds, unless extremely heavy fabrics are beingtreated.

After the acid treatment, the fabric is contacted with water whichimmediately dilutes the acid. During the acid treatment, the nylonfibers are swollen, distended, and become very tacky, but the nyloncoagulates immediately on contacting the water and the fibers in contactwith the nylon become bonded thereto. The fabric may be padded with theacid, to a controlled pick up and then immersed in water so that theconcentration of acid is reduced well below the minimum acid solvationconcentration described above. It is not necessary to wash all the acidfrom the fabric, and successive washings are not required.

A small quantity of nylon may become dislodged from the fabric in theacid and especially in the water bath where it appears as finelydispersed particles. In general the weight loss does not exceed about 5%by weight of the nylon. In addition, some nylon which becomessolubilized or dislodged by the acid, redeposits in the acid bath andserves as binder.

The washed fabric is treated next with an aqueous solution of alkali toneutralize any remaining acid. This is not an essential step, but isuseful because it prevents any further action by acid on the nylon asthe fabric is dried. During drying, as water evaporates, the acidresidue becomes more concentrated in the unevaporated water, ultimatelyreaching a concentration sufficient to swell the nylon. Byneutralization, this effect is avoided.

The choice of alkali to be used in the aqueous solution is not critical,but common alkali metal hydroxides and carbonates, or ammoniumhydroxide, carbonate or bicarbonate may be used. The concentration ofalkali is related to the speed of the fabric and pick up level, butpreferably these are correlated to achieve approximate neutrality as thefabric proceeds to the next stage which is a further wash to removesalts and any excess alkali.

At this stage, the nylon fibers may appear somewhat swollen andcoagulated nylon particles can be seen. Their flowability anddeformability have been increased substantially, but melting point doesnot appear to be affected appreciably. The acid treatment itself reducesthe strength of the nylon fibers to an enormous degree, but the fabricis held together by the entangled web of unaffected fibers. The strengthof the web is substantially restored during coagulation in water.

Following these liquid treatments, the fabric can be calendered orotherwise compressed with or without intermediate drying. Duringcompression, the fabric is heated to a temperature sufficient to furtherincrease the flowability of the nylon, generally to about 300 F., butthe temperature is insufficient to cause the other fibers in the blendto flow under the applied pressure. The pressure applied depends uponthe thickness of the fabric and its density but will ordinarily be about10 to 1000 p.s.i., preferably 10 to 500 p.s.i. Preferably thetemperature does not exceed 400 F.

The temperatures and pressures used in the acid and alkali treatmentstages, and preferably also coagulation, are conveniently roomtemperature. While heating the liquids might accelerate the process, solittle time is required that the advantage does not justify theadditional cost. Heating might also reduce the required acidconcentration, but any savings would be consumed by the cost of heating.The liquids also can be cooled, but this might slow down the process orrequire higher concentrations and is not desirable. Therefore, while anytem perature between the freezing and boiling points of the liquidsmight be used, it is preferred to allow the liquids to remain inequilibrium with the ambient temperature environment. However, duringwash stages, especially after the alkali treatment, it is preferable toheat the wash water to increase efficiency.

After the fabric is compressed, it is quite sturdy and uniform. Inparticular, a rather thick fabric can be split into two or more thinnerlayers which are substantially equivalent to each other, a clearindication of the removal of nonuniformities of structure.

The following examples illustrate the invention, all parts andpercentages being by weight.

Example I A needled bat weighing about 18 ounces per square yardcomposed of 42% nylon 66 fibers (3 denier x 1 /2" long), 23% polyesterfibers (2% denier x 1 /2" long) and 35% polypropylene fibers (1.8 denierx 1 /2" long) is made using conventional carding and needleloomequipment. A preformed bonded nonwoven carrier composed of 3.0 ounce persquare yard nylon 66 fibers (3 denier x 1 /2" long), 0.5 ounce persquare yard polypropylene fibers (1.8 denier x 1 /2" long) and 1.0 ounceper square yard natural rubber binder is centered within the structureby needle punching the batt uniformly through the carrier.

The fabric is needled to a density of approximately 3000 punches persquare inch using conventional 36 barb needles. Surface tufts arereadily visible on the needled batt and the diameters of the tufts whenviewed in cross section are undesirably large for certain end uses wheredensity uniformity is necessary.

After needling, the batt is dipped in a room temperature solution, whichcontains 40% by weight of 66 (96%) sulfuric acid (33 B.), at roomtemperature. The excess solution is squeezed out. The fabric is now weakbecause of the partial dissolving of the nylon fibers but sufficientlystrong to be pulled under light tension without undue distortion becauseof the presence of unaffected fibers. Dwell time in the 33 acid is 4seconds, although varying the dwell time does not appreciably affect thefinal properties. Upon leaving the acid, the fabric is immersedimmediately in a water bath at room temperature which congeals orcoagulates the partially dissolved nylon. Most of the acid is removed atthe same time. The strength of the fabric increases again. Next thefabric is immersed in a 10% by weight solution of ammonium hydroxide inwater for neutralization of the last traces of acid remaining. ,Ammoniumsulfate is formed as a by-product. The fabric is next given a finalwash, preferably in hot water, to remove the ammonium sulfate and excessammonium hydroxide. The fabric is then dried. Shrinkage of about 5-10%occurs fillingwise when the fabric is dried under tensionlessconditions, such as on dry cans.

At this stage, the flow and deformation properties of the nylon aredramatically altered, although its melting point is not materiallyaffected. The nylon fibers are adhered to each other and to some of theunaffected polyester and polypropylene fibers by the process. The nylonfibers also have a swollen appearance. The fabric is now strongly anduniformly bonded, and stiffer than the original batt, but notexcessively stiff.

Example II The fabric produced in Example I is compacted to the desiredgauge by hot pressing on a flat bed press. Temperature is about 250 F.and pressure required about pounds per square inch. The fabric afterpressing is very smooth and dense but not stiff. Surface tufts have beeneliminated. Cross section tufts also are barely discernable.

Tensile properties except elongation and tear remain virtually unchangedduring the bonding and subsequent densification. Elongation is reducedby 50% as is tongue tear. However, gauge, weight and stiffness areaffected. Leatherlike properties are greatly accentuated as a, result ofthe processing.

Microscopic examination shows that the nylon fibers have been deformedconsiderably and bond the unaffected fibers.

Example III A batt of 60% polyester fibers (1 /2 denier x 1- /2 incheslong) 40% nylon, 66 fibers (1 /2 denier x 1 /2 inches long) is needledabout 4000 punches per square inch using conventional needlingequipment. Total weight is about 10 ounces per square yard.

The fabric is then passed through an acid bondingneutralization-washline using an acid solution containing of 66 (96%) sulfuric acid inwater and the neutralization is effected with aqueous ammonia in thebath immediately following the acid bath.

After bonding the fabric is pressed to about 25 mils on a flat bed pressat 275 F. forone minute. The resulting fabric is smooth and well bonded.Needle marks are barely discernible and the fabric is suitable as asubstrate for vinyl or urethane coating to make shoe upper and similarmaterials.

Example IV A carded and crosslaid web weighing 4 ounces per square yardcomposed of 60% polyester fibers (1 /2 denier x 1% inches long) andnylon 66 fibers (1 /2 denier x 1 /2 inches long) is needled into eachside of a 1.0 ounce per square yard polyester scrim. The fabric isreneedled about 2500 punches per square inch using conventional needlingequipment.

The fabric is next passed through the acidbondingwash-neutralization-Wash line described in Example I except thatthe acid tank contains 35% fluoboric acid in water, and the last rinsetank contains hot water at about 150 F.

After the pass through the acid bonding line, the fabric is hotcalendered at 300 F. and adequate pressure to cause the coagulated nylonto flow and bond to adjacent polyester fibers. A smooth well bonded,strong but not excessively stifi'. fabric results which is suitable forvinyl coating.

Example V Example VI A carded and crosslaid web weighing 4 ounces persquare yard composed of polyester fibers (2 A denier x 1- /2 incheslong) and 40% polypropylene fibers (1.8 denier X 1 /2 inches long) isneedled into each side of 1% mil .015") nylon film. The fabric isreneedled to a density of 3,000 punches per square inch.

The fabric is then passed through the acidbondingwash-neutralization-wash line described in Example I with 38%aqueous sulfuric acid (specific gravity=1.28) at room temperature in theacid tank.

The fabric then is calendered at 300 F. and adequate pressure to causethe coagulated nylon to flow and further bond to adjacent unaffectedfibers. A smooth, wellbonded strong but not excessively stiff fabricresults, which is suitable for vinyl coating.

Example VII A carded and crosslaid web weighing 4 ounces per square yardcomposed of polyester fibers (3 de nier x 1% inches long) and 25% nylonfibers (2% denier x 1% inches long) is needled into each side of nylonfilm (1 /2 mil thickness). The fabric is reneedled to a density of 3,000punches per square inch.

The fabric is then passed through the acidbondingwash-neutralizatiomwash line described in Example I with 38%sulfuric acid (specific gravity=1.28) at room temperature in the acidtank.

-After acid bonding, the fabric is calendered at 300 F. and adequatepressure to cause the coagulated nylon to flow and further bond toadjacent unaflfected fibers. A smooth, well-bonded strong but notexcessively stiff fabric results which is suitable for vinyl coating.

Example VIII Three 3.5 ounces per square yard webs composed of 75%polyester fibers (3 denier x 1% inches long) and 25% nylon fibers (2%denier x 1% inches long) made on conventional carding and crosslayingmachinery and lightly needled are needled together (combined) in onepass through a needle loom. The fabric is then reneedled to a density of3,000 punches per square inch.

The needled fabric is passed through an acidbondingwash-neutralization-wash line as described in Example I exceptwith 38% aqueous sulfuric acid (specific gravity=1.28) at roomtemperature in the acid tank.

After acid bonding, the fabric is calendered at 300 F. and adequatepressure to cause the coagulated nylon to flow and further bond to theadjacent polyester fibers. A smooth, well-bonded strong but notexcessively stiff fabric results which is suitable for vinyl coating.

I claim:

1. A process for the manufacture of a bonded nonwoven fabric comprisingcontacting, with an aqueous solution containing at least 25% by weightof a strong acid, a nonwoven fiber web which comprises a blend of nylonstaple fibers and staple fibers of another material which issubstantially unaffected by said acid contacting said web with water todilute the acid and coagulate the nylon, thereby increasing theflowability of the nylon fibers, and compressing said web so that thenylon bonds the fibers of said other material to each other.

2. A process as set forth in claim 1 in which the nylon is selected fromthe group consisting of nylon 6 and nylon 66.

G. A process as set forth in claim 1 in which the other material isselected from the group consisting of polyethylene terephthalate,polyolefin, glass, cellulose, polyvinyl chloride and Saran.

4. A process as set forth in claim 1 in which the strong acid has adissociation constant in water at 20 C. of at least 10- 5. A process asset forth in claim 4 in which the dissociation constant in water at 20C. is at least 6. A process as set forth in claim 4 in which the strongacid is selected from the group consisting of sulfuric acid, nitricacid, phosphoric acid, hydrochloric acid and zfluoboric acid.

7. A process as set forth in claim 6 in which the strong acid issulfuric acid and the concentration is 30 to 50% by weight.

8. A process as set forth in claim 6 in which the strong acid is formicacid and the concentration is at least 75% by weight.

9. A process as set forth in claim 6 in which the strong acid isfiuoboric acid and the concentration is about 35% by weight.

10. A process as set forth in claim 6 in which the strong acid isphosphoric acid and the concentration is about 50% by weight.

11. A process as set forth in claim 6 in which the strong acid isglacial acetic acid.

12. A process as set forth in claim 1 in which the water contacted withthe nonwoven web is in the form of an alkaline solution which dilutesthe acid and coagulates the nylon and also neutralizes the acid.

13. A process as set forth in claim 1 including the further step oftreating the water-washed web with alkali to neutralize residual acid.

14. A process as set forth in claim 13 in which the alkali treatment iscarried out by contacting the fabric with an aqueous solution of a watersoluble alkali.

15. A process as set forth in claim 13 including the step of contactingthe web with water after neutralization to remove residual alkali andsalts.

16. A process as set forth in claim 15 in which the water wash afterneutralization is carried out with hot water.

17. A process as set forth in claim 16 in which the temperature of thehot water is at least about F.

18. A process as set forth in claim 1 in which the nonwoven fiber web isneedle punched prior to contact with said aqueous solution, the needlepunching creating surface irregularities which are removed in the acidtreatment.

19. A process as set forth in claim 18 in which said needle punching is1000 to 20,000 punches per square inch.

20. A process as set forth in claim 1 in which the pressure is 10 to1000 p.s.i.

21. A process as set forth in claim 1 in which pressure is applied bycalendering.

22. A process as set forth in claim 1 in which the nonwoven fiber web isa laminate of at least one batting layer and a sheet-like carrier layer.

23. A process as set forth in claim 22 in which the carrier layer is ascrim.

24. A process as set forth in claim 22 in which the carrier layer is aplastic film.

25. A process as set forth in claim 24 in which the plastic film isnylon which also is treated with said strong acid.

26. A process as set forth in claim 1 in which said strong acid is atroom temperature when contacting said web.

References Cited UNITED STATES PATENTS 2,730,479 1/ 1956 Gibson 161DIG 2FOREIGN PATENTS 882,953 11/1961 Great Britain 156-316 BENJAMIN R.PADGETT, Primary Examiner U.S. Cl. X.R.

156-622, 62.8, 79, 148, 307, 316, 317; l6l-DIG 2

